The vulcanisation behaviour of a rubber mixture may be represented for example by means of a Vulcameter Curve. The Vulcameter curve is formed by plotting the torques (moment of rotation), determined with a Vulcameter as described in DIN 53 529 (German Industrial Standard 53529), on the abscissa of a rectangular co-ordinate system against the vulcanisation time on the ordinate. The entire disclosure of DIN 53529 is hereby incorporated by reference and relied upon. The onset of vulcanisation is shown by the ascending branch of the vulcameter curve. Thereafter the curve normally reaches a maximum or indicates a maximum value and, in most cases, descends more or less quickly or slowly as vulcanisation continues. Vulcameter curves recorded at a constant temperature may also be called crosslinking isotherms.
In the vulcanisation of rubber mixtures based on natural or synthetic rubbers in the presence or absence of rubber fillers by means of sulphur and vulcanisation accelerators, as normally practised in industry, these crosslinking isotherms normally pass through a maximum which is formed as a result of the fact that, during the complex chemical processes occuring during vulcanisation, the build-up of polysulphidic crosslinks between the rubber molecules predominates in the initial stage, whereas in the final stage the degradation of intermolecular polysulphidic and disulphidic bridge bonds to intramolecular heterocyclic rings occurs in the final stage. In a crosslinking isotherm obtained by vulcametry, these phenomena are shown by a continuous downward trend of the crosslinking isotherm, i.e. in a fall in the torque (moment of rotation) values and, in practice, the moduli decrease with increasing vulcanisation time. The change in the relative crosslink density of the vulcanisate (level of the torques) and the relative crosslink density present at any stage of the vulcanisation proces may be read off from the trend of the vulcametrically determined crosslinking isotherm. The change in the crosslink densities is accompanied by a change in the mechanical properties of the vulcanisates (where this property is dependent upon the crosslink density), such as their tensile strength, breaking elongation (elongation at break), elasticity, Shore hardness, heat build up, abrasion, etc.
In practice, the downwardly sloping branch of the vulcameter curve signifies a deterioration in the above mentioned properties of the vulcanisate. This phenomenon is technically known as "reversion". On account of the change in the mechanical properties of the vulcanisates by which reversion is accompanied, reversioning vulcanisates are generally undesirable. This applied in particular in the production of thick-walled rubber articles because the poor thermal conductivity of articles such as these in their individual discrete regions (volume elements) can give rise to differing mechanical properties which means that, on completion of vulcanisation, the vulcanisate is not homogeneous in regard to its crosslink density. In the case of thick-walled rubber articles, the appearance of reversion necessitates a distinct reduction in the vulcanisation temperature in order to postpone the onset of reversion. Another notorious phenomenon is that reversion increases with increasing temperature. Any reduction in temperature during the vulcanisation of thick-walled articles results in a commensurate increase in the heating times. For example, the heating times for heavy duty off the road tires ranges from about 10 to 14 hours at a vulcanisation temperature of 120.degree. C.
Now, a significant aim which the present invention sought to attain was to enable the vulcanisation temperatures to be increased without any adverse effects on the properties of the vulcanisates, i.e. to avoid unfavourable reversion phenomena and considerably to shorten the heating times (vulcanisation times) so that the production facilities may be distinctly better utilised and a faster output or higher productivity is achieved.
It is known that oligosulphidic silanes may be used in the vulcanisation of rubber mixtures with sulphur to make the mixtures with silicate fillers added thereto easier to process and to obtin vulcanisates equivalent or superior in quality to vulcanisates obtained from mixtures filled with carbon black (cf. German Pat. No. 2,255,577 or Thurn U.S. Pat. No. 3,873,489). The entire disclosure of the Thurn U.S. patent is hereby incorporated by reference and relied upon. A typical representative of these oligosulphidic silanes is 3,3-bis-(triethoxysilylpropyl)-tetrasulphide or the commercial product Si 69.
It is also known from German Offenlegungsschrift No. 2,536,674 and related Wolff U.S. application Ser. No. 34203, filed Apr. 27, 1979, now U.S. Pat. No. 4,229,333 that rubber mixtures containing silicate fillers can be crosslinked solely with oligosulphidic silanes and vulcanisation accelerators, i.e. without elemental sulphur or sulphur donors. In this case, mixtures of silica and carbon black are advantageously used as fillers. The entire disclosure of German OS 2,536,674 and the Wolff application are hereby incorporated by reference and relied upon.
Crosslinking agents with which attempts have been made to avoid reversion phenomena, particularly in the case of reversion-prone rubbers, such as natural rubber and polyisoprene, have long been known in the rubber-processing industry. These crosslinking agents are, for example, peroxides which lead to --C--C-crosslinking(carbon-carbon cross-linking) or thiuram disulphides which form --C--S--C--bridge bonds. Accordingly, the development of polysulphidic degradable crosslinks as described above is avoided. Systems of the type in question also include vulcanisation systems using so-called sulphur donors which, in terms of function, are distinguished by the fact that, once again, no polysulphidic crosslinks are formed in contrast to standard sulphur vulcanisation. They also include crosslinking systems whose function is based on the fact that, where basically crosslinking accelerators are used, crosslinking is controlled by the addition of small quantities of sulphur in such a way that predominantly monosulphidic bridge bonds, i.e. bridge bonds which cannot be further degraded, are formed.
However, the avoidance of polysulphidic crosslinks is also associated with changes in the properties of the vulcanisates which are undesirable. For example, the tensile strengths and breaking elongations (elongations at break) are reduced for the same crosslink density by comparison with sulphur vulcanisates and, what is more important, the tear initiation and tear propagation resistance is drastically reduced. One particularly unfavourable aspect of this method of vulcanisation is the increase in damage to vulcanisates, for example in the form of chipping and chunking effects, which seriously restricts the use of systems such as these so that it is better to work at a low vulcanisation temperature with conventional sulphur/accelerator systems, i.e. to accept and mimimise the reversion phenomena.
The above mentioned vulcanisation systems which impart resistance to reversion are even less applicable to rubber mixtures containing silicate fillers or mixture of carbon blacks with silicate fillers. The silicate fillers disrupt in particular with these vulcanisation systems to such an extent that an adequate crosslink density cannot be obtained, even by using the crosslinking agents in very large quantities.
Accordingly, the use of the described crosslinking systems which impart resistance to reversion is severly restricted and confined to special types of rubber, and cannot or can only be used to a very limited extent, in those rubbers normally used for a wide range of applications, such as natural rubber and styrene-butadiene rubbers.
Accordingly, another problem which the present invention sought to solve was to find a crosslinking system which could be used in as many types of rubber as possible, preferably in natural rubbers and polyisoprenes, with the object of producing therefrom vulcanisates which do not have any of the numerous unfavourable properties associates with reversion.